Project Summary/Abstract Our brains carry out complex cognitive tasks via the intricate electrical communication between neurons. The specialized site where this communication occurs is the synapse, and it is capable of various forms of plasticity that enable the constant and adaptive refinement of the communication between neurons. The amount of neuronal activity that is transmitted at the synapse typically dictates the type of modification, and raising the level of communication between neurons strengthens their synaptic connection through an adaptive increase in the number of connections. This process is collectively known as activity-dependent synaptic plasticity, and is thought to be the cellular foundation for learning and memory. The long term goal of this project is to better understand the basic cell biological roots of activity-dependent synaptic plasticity. In particular, I am interested in understanding the functional role of intracellular mitochondrial trafficking in supporting the growth of synapses adapting to increased neuronal activation. I will carry out this project using Drosophila Melanogaster larvae, or fruit fly maggots, as an experimental model system that was chosen due to the powerful genetic tools available. Like motor neurons in our spinal cord, the muscles in the body wall of the larvae are innervated by motoneurons that are responsible for sending the signal when to move. These motoneurons form a synapse onto the muscle, which is known as the neuromuscular junction (NMJ) synapse. The presynaptic terminal of the NMJ synapse will undergo activity- dependent plasticity upon increased activity of the motoneurons, from increased movement of the larvae. I can microscopically image this synapse directly through the cuticle (skin) of the larvae using technology developed in my lab; including its changing shape and intracellular dynamics (such as changes in mitochondria) that occur simultaneously with the growth when neuronal activity is increased. My central hypothesis is that neuronal activity induces the formation of acute synaptic growth that is eventually stabilized by the trafficking of mitochondria into this nascent growth, which facilitates its long-term maturation to becoming a mature synaptic connection. Using the tools I described, I will test this hypothesis with two specific aims: (1) I will use a genetic mutant larvae in which the trafficking of mitochondria to the NMJ is dysfunctional to see if any aspects of activity-dependent growth are successful, hence pinpointing a precise role for mitochondrial trafficking in the process. (2) I will seek to functionally characterize a molecular mediator responsible for promoting activity-dependent synaptic growth, and determine whether the driving force for its ability to promote the growth of synapses is in its regulation of mitochondrial trafficking. The insight gained from this work will uncover new knowledge on the cell biology of synaptic plasticity and mitochondrial trafficking in neurons, two processes that commonly result in neurodegenerative diseases when dysfunctional.